1. Technical Field
The present invention relates to electrical interconnections. More particularly, the present invention relates to electrical interconnections that are formed using particle enhanced joining of metal surfaces.
2. Description of the Art
Interconnect technology has not kept pace with microminiaturization in the electronics industry. Thus, component packages and the connectors used to form an electrical and/or mechanical interface between various components and assemblies in electronic products are now the most expensive portion of such products. Component packages, connectors, sockets, plugs, and the like are also the bulkiest and heaviest portion of such products.
Conventional interconnect technology complicates the electronic equipment design and manufacturing processes by introducing special considerations into such processes with regard to component placement, heat generation, power loss, and signal propagation delay. In this way, potential gains in performance, realized by new and emerging technologies, are not fully exploited. Rather, such advances are negated or held back by such considerations as are mentioned above.
Conventional interconnect technology provides different approaches to interconnecting electronic components (each approach having variations that do not need to be discussed herein): the use of solder reflow to make permanent, low-ohmic connections; the use of wiping contacts to make temporary, medium-ohmic connections; the use of filled adhesives to make permanent, medium to high ohmic connections; and the use of sheet materials and other exotics to make medium to high ohmic connections. Each of these technologies is either approaching obsolescence in view of the continual production of improvements in electronic components, such as integrated circuits, or it has certain shortcomings that render its use limited or unreliable.
Solder reflow technology produces a metallic contact of moderate strength, but requires that the bonded surfaces (and surrounding areas) be subjected to high heat. Such thermal stress tends to weaken or damage the components joined and therefore results in higher initial and long term failure rates. Solder bonds are easily fractured under moderate stress and, if the bonds are not formed under strictly controlled conditions, they are subject to producing cold (i.e. high ohmic, low mechanical integrity) connections.
To form a bond using solder reflow it is necessary to prepare the surface to be bonded with highly corrosive and environmentally hazardous fluxes. After the bond is formed, such fluxes must be cleaned from the surface of the electronic assembly.
Although solders are known that do not require cleaning after a bond is formed, such solders either require special atmospheric conditions to be used, or they must be used in conjunction with special fluxes. Thus, such alternative soldering approach is both expensive and of uncertain reliability.
Solder bonds cannot be formed in a uniform fashion. For example, voids often occur within the bond. Thus, solder bonds must be visually inspected after the bond is made. If the bond is inadequate, it must be reworked, if possible. Such rework is a labor intensive activity that runs the risk of damaging the circuit board and neighboring components on the circuit board.
To use solder reflow technology it is also necessary to purchase and maintain several large pieces of expensive machinery and test equipment. Accordingly, solder reflow technology requires several process steps and implementing equipment, adding to the expense of manufacturing electronic products, while unnecessarily limiting plant throughput.
A solder bond is a permanent bond, and reworking a circuit assembly to remove and replace a defective component subjects the entire assembly, and especially the area around the defective component, to elevated temperatures, which results in additional thermal stress. The solder contacts themselves take up significant space on a circuit board and thus artificially limit the absolute component density that would otherwise be possible, for example on a circuit board. As the solder used in solder reflow is only available in a limited number of materials, the continuing introduction of incompatible materials means that there are fewer designs to which solder reflow can be applied.
Wiping action interconnect technology (e.g. sockets, plugs, needle pins ect.) forms a temporary electrical interconnection and thus readily allows the remating of various components and assemblies (for example, when replacing a defective component). A problem with using such technology is that it is subject to the persistent formation of oxides along a contacting surface, which increases contact resistance. In time, these oxides build up, hastening connection failure and thus equipment failure.
Wiping action technology is only available in the form of various connectors and sockets, etc. These devices usually provide a contact surface formed from a limited range of special metals, alloys, and other expensive materials as are suitable for maintaining a sliding connection. The devices themselves have interfering electrical properties due to their size, orientation on a circuit board, etc. and thus degrade signal propagation through the interconnect (by introducing resistive, capacitive, and inductive components into the signal path).
Wiping action technology provides high ohmic connections that produce excessive and unwanted heat, and therefore contribute to equipment failure while wasting energy. The wiper mechanism, for example a socket, requires significant space on a circuit board. Thus, potential circuit operating speeds are degraded due to propagation delays (i.e. the time it takes a signal to traverse a greater distance). Sockets, plugs, and the like are only available in a limited number of configurations and materials. Thus, the evolution of electronic technology is constrained by the limitations wiper interconnects impose upon a designer.
Additionally, wiper interconnects are highly unreliable. A punishing environment, for example one subject to intense vibration, temperature extremes, and/or high levels of contamination (e.g. that in which a laptop computer is used), tends to disrupt the continuity of connections made at a wiper interconnect. As wiper interconnects are mechanical devices they corrode and are subject to wear. Thus, they only have a limited useful life.
Filled adhesives generally provide a binder and a conductive filling, such as silver or gold. These materials are unsuitable for most interconnect applications because they form permanent medium to high ohmic connections.
Sheet materials and other exotic interconnect media are largely untested in most interconnect applications. Accordingly, the reliability of such materials is questionable. Typically, a sheet material provides an elastic matrix having the ability to form spaced conductive pathways therethrough. Conduction in such materials is provided by a string of conductive particles, microwires, and the like. Such materials are expensive, show temperature induced changes in physical properties (i.e. they are rigid when cold and soften when heated), are prone to shorting and therefore unreliable, and they typically provide only medium to high ohmic connections.
Examples of such exotic interconnect technology include the following: 3M Corporation of Minneapolis, Minn. provides a material consisting of diamond particles in a polyamide binder; Nitto Denko Company of Japan provides a circuit board material having conductive bumps on either side which are interconnected by vias and which form a connection under conditions of extremely high pressure; and Digital Equipment Corporation of Maynard, Massachusetts provides silicon backed polyamide thin film deposited metal circuit boards.